1. Field of the Invention
The present invention relates generally to methods for operating a series hybrid vehicle and, more specifically, to methods for maximizing fuel efficiency while minimizing disruptions in drivability.
2. Description of the Related Art
The term “hybrid vehicle,” in the broadest sense, denotes a vehicle having more than one power source and one or more energy storage means. The goal of a hybrid vehicle is to combine several similar or dissimilar types of energy stores and/or energy converters with different drive components, and operate each power source under varying operating conditions in a manner that results in greater overall energy savings than would otherwise be achieved through the use of a single power source.
The primary power source of a hybrid vehicle is usually an engine powered by fuel energy (primary energy), and the secondary power source is usually, but not limited to, one or more electric motors/generators powered by electric energy (a form of “secondary energy”) and/or one or more hydraulic motors/pumps powered by hydraulic pressure (also a form of “secondary energy”).
When the drive components of a hybrid vehicle allow the vehicle's primary and secondary power sources to both independently transmit power to the vehicle's wheels, the vehicle is commonly referred to as a parallel hybrid vehicle and the wheels of the vehicle can be driven solely by an engine (as is done with conventional vehicles), or solely by the secondary power source. In contrast, when the drive components of a hybrid vehicle are configured such that only the vehicle's secondary power source transmits power to the vehicle's wheels, the vehicle is commonly referred to as a series hybrid vehicle. In series hybrid vehicles, the engine is used to convert energy and provide power with which to power the secondary power source, but the engine is not mechanically linked to the vehicle's wheels.
To date, parallel hybrid vehicles have been more commercially successful than series hybrid vehicles. For example, the Insight, a hybrid vehicle manufactured by Honda Motor Company, and the Prius, a hybrid vehicle manufactured by Toyota Motor Corporation, represent the first two mass-marketed hybrids, and both are parallel hybrid vehicles.
The commercial success of parallel hybrid vehicles over series hybrids is, in large part, due to the state of technology and knowledge that have been available with respect to energy storage devices used for storing a hybrid vehicle's secondary energy. For example, many of the first generation secondary energy storage devices, such as early generation batteries, require a low charge rate in order to preserve the life of the energy storage device. This low charge rate requirement restricts the design choices available to a hybrid vehicle designer and, in particular, restricts the choices available for a series hybrid more than it restricts the choices available for a parallel hybrid. In series hybrid vehicles, the charge rate is, by definition, provided by an engine. Thus, design choices affecting the size and calibration of an engine in a series hybrid vehicle employing previous generation energy storage devices are limited by the need to have the engine of a series hybrid produce a low enough power level to generate the required low charge rate, while still achieving greater overall energy savings from the hybrid design than would otherwise be achieved through the use of a single power source.
Since engine efficiency is better at high loads than at low loads, engines in prior art series hybrid vehicles are typically very small, and are calibrated to operate at high loads. This allows the engine to operate closer to its maximum efficiency level while still producing a low enough power level to generate the required low charge rate. However, due to the low charge rate, the energy stored within previous generation energy storage devices is often used up more quickly than it can be replenished. Thus, when the energy stored within the energy storage device of a series hybrid vehicle is depleted, the driver is unable to complete a trip because the engine alone is too small to safely propel the vehicle.
As a result, there is a need for a new and improved method of operating a series hybrid vehicle.